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Potted transformers

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Thanks for you expertise BudP,
It's mounted on a piece of wood for the time being, so it affect on the chassis can be ruled out. When I grab the endbells the noise level goes down quite a bit. For some reason this transformer buzzes a significant amount more than most. When I have it hooked to a variac it's quiet till it gets within 10V of mains voltage then it starts humming like a MoFo. Do transformers hum a significant amount more depending on the load? It's used in a bridge rectified config and it's idle current draw is about 1/6th of what the transformer is rated for. However, the transformer isn't center tapped and it's being used for a +/- rails. Would adding a snubber help?
 
Bud,

Very sincere thanks for a lot of info not normally easily avaiable. One understands better after that.

My own problem is with C-core transformers - OK, I know I should know better, but thus far (out of 12) I had only one buzzing sample (they are 500VA double loop). I have not yet discussed with my winder, but wondered whether some varnish/whatever coating over the C ends just before clamping together would give enough "seepage" into laminations to cure this.

Thanks.
 
Jeb,

If you can get the end bells off, not always possible, I would find a rubber puck and insert it in between the coil and end bell. Moderate stiffness rubber, so, from an isolation cushion for small machinery or the gasket for a camper top for pickup trucks, cut down to a 1/2" dia or so.

Also check out wedging with wooden or plastic wedges, driven in between coil and core. This is the most common way to silence power growl. Even this is not 100% effective.

You could also check exciting current (no load AC current) against your other transformers, just to make sure the core was fully closed before vac varnish. Should not be more than 10% above a quiet one.

Sometimes there is just no reason to be found, but you are correct about the end bells being a great amplifier for hum.




Johan,

Nothing wrong with C Core power transformers, so long as the bands are tightened correctly. If the metal is an .014" thick M6, then a super thin, super glue might do the trick. If it is a thinner material, then one of the two part, extremely low viscosity epoxies are the only thing to use. You do not want a build up of material in the gap and regular varnish is unlikely to wick in completely, if at all.

Another possibility is wooden wedges slipped in between coil former and core, on both sides of the bobbin. Not always possible, but there might be enough space. Even flat plastic toothpicks might do what is needed. This is only on the wide surface of the core material, not on the edges of the "stack".

Bud
 
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